1. Field of the Invention
The present invention relates to a method for forming a speaker cone plate, more particularly a high frequency speaker cone plate.
2. Description of the Prior Art
Speaker cone plate generally requires a high Young's modulus and a large propagation speed, but particularly in a high frequency speaker cone plate, rigidity and surface hardness are important factors for improving the high frequency characteristics.
Conventionally, the rigidity of the speaker cone plate has been improved by forming a hard layer for example, of an aluminum oxide, silicon carbide, silicon oxide or boron carbide on the surface of speaker cone plate.
However, in such conventional methods in which the hard layer is simply formed by vacuum evaporation, ion plating, thermal spraying or electroplating on the surface of the speaker cone plate, the hard layer may show satisfactory performance for example, strength-to-weight ratio, but the substrate of the speaker cone plate is unable to show enough strength per weight, so that a satisfactory strength cannot be achieved over the entire speaker cone plate.
Also a metal substrate with a hard layer formed thereon by ion plating is associated with a drawback of pores remaining on the surface and in the interior of the speaker cone plate. Besides, in this method, the evaporation of hard layer onto the substrate stimulates anisotropic vertical crystal growth of the evaporated material, which tends to give rise to deterioration of bending, torsion and shear strengths, eventually leading to cracking and tipping of the speaker cone plate.